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WORKING OF TWO STROKE CYCLE S.I ENGINE

 The working principle of two stroke S. I Engine is describes as follows.




1. FIRST STROKE ( SUCTION AND COMPRESSION ) :

 The first stroke consists of the suction and compression processes. During the first stroke , the piston moves upward from BDC to TDC. When the piston is at BDC, the partially compressed air fuel mixture from crank case enters into the cylinder through transfer port. The piston moves upward and compress air contained in it till the piston reaches TDC. At the end of compression stroke, the spark plug  produces spark, it will ignite the compressed high pressure fuel air mixture. ]

2. SECOND STROKE ( EXPANSION OR POWER AND  EXHAUST STROKE ): 

When air fuel mixture is ignited, both the pressure and temperature of  the products of combustion will suddenly increase . Therefore, the piston receives power impulse from the expanded gas and it pushes the piston downward and also produces the power stroke. During the expansion stroke, some of the heat energy produced is converted into mechanical work. 

During downward stroke of piston, already entered air fuel mixture in the crank case is partially compressed by the underside of the piston. This pre compression process is called crank case compression. At the end of the power stroke, the exhaust port opens and burnt gases are sent out of the engine through this port.

At the same time, all the burnt gases are not exhausted. Some portion of it will remain in the cylinder. when the piston moves to BDC, The fresh air fuel mixture from crank case enters into the cylinder to sweep out the burnt gases. The process of sweeping out the exhaust gases with the help of fresh air fuel mixture is known as scavenging. The scavenging helps to remove the burnt gases from the cylinder.  

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